Polyurethane foam production involves the accurate pumping, mixing, and dispensing of several components or streams into a mold or onto a moving conveyor belt. The number of streams can be from two to seven; however, the typical formulation is composed of two streams consisting of an isocyanate stream and a resin stream. The resin stream is a mixture of polyether or polyester polyols, cross-linkers (e.g., diethanolamine), surfactant, catalyst, water, auxiliary blowing agents, and other additives. The isocyanate stream comprises toluene diisocyanate (TDI), various forms of diphenylmethane diisocyanate (MDI), or a mixture of the two.
A superior quality flexible molded foam displays several important characteristics. It will have good bulk, vent, and shear stability which implies the foam has a small, uniform cellular structure throughout the interior of the foam. These foams will also display good surface stability, defined as having a layer of fine cells adjacent to the outer surface of the foam, and good dimensional stability (i.e., exhibit a reduced tendency to shrink after being removed from the mold). Foams that are less susceptible to shrinkage will be easier to process, require less mechanical crushing which can weaken the physical integrity of the polyurethane, and have lower scrap and repair rates. Superior quality non-molded flexible foams primarily require good bulk dimensional stability, which if absent will lead to foam collapse or severe densification. Reducing the overall emission of additives from a flexible foam is also desirable, for this can lead to reduced automotive windshield fogging as an example.
The manufacturing equipment and chemicals have an important effect on the quality of the foam; however, the surfactant is often one of the most critical components of the formulation as it has a direct and significant influence on the bulk, vent, shear, surface, and dimensional stability as well as the emissions of the foam.
In the past, chemical strategies for selecting formulation variables in order to optimize bulk, shear, vent, surface, and dimensional stability have been successful for many polyurethane foam applications. Key variables include the judicious selection of surfactants and catalysts, and the incorporation of cell opening polyols.
The foam industry is now facing cost reduction issues, and is challenged with maintaining foam physical properties while at the same time reducing their raw materials and processing costs. Approaches include reducing foam density by incorporating more water in the formulation or injecting liquid carbon dioxide, lowering the amount of relatively expensive graft copolymers, using blends of TDI/MDI, and incorporating isocyanate terminated prepolymers. All of these approaches have placed increasing challenges on the accompanying additives, particularly in terms of maintaining foam dimensional stability.
Silicone surfactants used for the production of flexible polyurethane foams are typically polydimethylsiloxane (PDMS) fluids and/or organomodified PDMS fluids such as siloxane polyether copolymers.
The PDMSs used in flexible polyurethane foams, including high resiliency (HR) foams, generally are a mixture or distribution of straight-chained or branched, fractionated PDMSs with chain lengths DP (DP=n+2, where n is the number of dimethylsiloxane units) ranging from 5 to 20.
U.S. Pat. No. 4,139,503 discloses a process for making high resiliency polyurethane foams using specific siloxane components. Polydimethylsiloxanes with DP less than 7 (n&lt;5) are taught as ineffective but having no adverse effect while PDMSs with DP greater than 20 (n&gt;18) have a highly undesirable defoaming or antifoaming effect. Using PDMS fluids containing the wide spectrum of molecular weights disclosed by U.S. '503 will reduce the openness of the foam and lead to shrinkage.
U.S. Pat. No. 4,042,540 discloses making highly elastic soft polyurethane foams in the presence of certain low molecular weight organopolysiloxanes, including PDMSs. The DP of the PDMS is 4 to 12 (n=2-10), while 4 to 10 (n=2-8) is preferred, and 6 to 8 (n=4-6) being especially preferred. Higher molecular weight PDMS species should only be present in very small amounts, as DP higher than 9 (n&gt;7) leads to a noticeable increase in the tendency of the foam to shrink. Lower molecular weight products can be used in a mixture without objection. Example 4 of U.S. '540 demonstrates that a mixture of PDMSs having a DP span of 5 to 9 (n=3-7) made a non-shrinking foam, while a mixture of PDMSs having a DP span of 8 to 14 (n=6-12) lead to shrinkage. U.S. '540 also discloses that pure molecular weight cuts of PDMS fluids with a DP below 8 (n&lt;6) have a different efficacy for preventing foam shrinkage.
U.S. Pat. No. 4,347,330 discloses making high resilience open celled flexible polyurethane foams using three cell modifiers consisting of a polysiloxane-polyoxyalkylene copolymer, a polymethylsiloxane, and a polyether polyol cell modifier containing polyoxyethylene groups.
U.S. Pat. No. 5,633,292 discloses the production of high resilience polyurethane foams using particular polysiloxane cell stabilizers.
There are many references that teach the use of organomodified, or organofunctional, PDMSs in flexible foam. This class of silicone surfactant is always required in formulations where the resulting foam will collapse when no silicone is present. Generally these structures will stabilize the foam but will also give rise to poor dimensional stability.